In the new comedy “What If,” opening Friday, Daniel Radcliffe and Zoe Kazan play best friends with a charming will-they-or-won’t-they spark. In real life, Radcliffe, the 25-year-old Brit best known for playing Harry Potter on film, and Kazan — the 30-year-old star of indies such as “Ruby Sparks” and the granddaughter of legendary film director Elia (“On the Waterfront”) — are pals, too. It’s strictly platonic, though: Radcliffe’s dating actress Erin Darke; Kazan’s been in a relationship with actor Paul Dano for seven years. Not that that’s stopped them from bonding over the NFL and Broadway . . .

Zoe, were you a Harry Potter fan?

Zoe Kazan: I read all the books . . . by the time the movies started coming around, I was Harry Potter’d out. I saw a few of them.

Daniel Radcliffe: Sometimes it’s embarrassing that a lot of people have seen the movies many more times than me and know the movies a lot better than I do. I’ve seen all of them two or three times at premieres and screenings and haven’t watched any of them again since.

How did you two meet?

ZK: I had met Daniel [in 2008] when he was doing “Equus” on Broadway. I was doing another show [“Come Back, Little Sheba”] and we had a friend in common. I heard all of these amazing things about him. I went in more impressed personalitywise than Potter-wise.

Did you do any off-screen bonding sessions to establish your chemistry for “What If”?

ZK: We were staying in the same hotel [in Toronto]. We did a lot of hanging out and getting room service. I just remember watching a lot of American football, because Dan is into that.

DR: She introduced me to lots of movies. She’s . . . much higher-brow than me.

In the film, your characters are friends first. Do you think it’s a good idea to be friends with someone before dating them?

ZK: I’ve never been in that situation. I always had a gut reaction. I’ve always gone for it. That probably doesn’t say anything good about my character.

What drew you to the film?

DR: [In the script], there was an exchange between the [characters] about the words “fore” and “forte” — the pronunciation and the etymology, and I was like, OK, I know this guy, I kind of am this guy. It would be sort of fun to get to play that side of myself.

Dan, you’ve made New York your home away from home in recent years. Why’s that?

DR: People are much nicer and more cheerful than in London. I love London, but we can be grumpy. And . . . at a time when everyone in the world was determined that I would only be Harry Potter, New York and the theater community welcomed me and gave me a chance to prove otherwise.

And Zoe, how did you end up in Brooklyn?

ZK: Seven years ago I was living in the East Village with my roommate, and our apartment got totally infested with mice. So she found a place in Carroll Gardens, and we moved out there. It was kind of impulsive. But now I have a lot of friends in the neighborhood, and I got Paul to move out here with me. Paul loves Henry Public, so we go there all the time. We get drinks, and he always orders the turkey sandwich, then I sneak food off his plate.

Dan, how about your free time? Is it true you’re a karaoke fan?

DR: I’ll confess to that. I often try to do Muse’s “Feeling Good” and some Frank Sinatra, Bobby Darin. Me and my friend tried to sing “I Get Around” [by the Beach Boys], but it’s way harder than you think it’s going to be.